FI82200B - FOERFARANDE OCH ANORDNING FOER AOTERVINNING AV I GASER BEFINTLIGA FOERORENINGAR, ISYNNERHET LOESNINGSMEDEL. - Google Patents

Description

82200

Method and apparatus for recovering impurities in gases, especially solvents

The present invention relates to a process for recovering impurities in gases, in particular solvents, according to the preamble of claim 1. According to such a process, compounds are separated from gases by adsorption on activated carbon, from which they are desorbed by raising the temperature of the activated carbon layer.

The invention further relates to a device according to the preamble of claim 4.

New laws and regulations in air protection have made the recovery of industrial solvents from exhaust air very topical.

Water-insoluble solvents such as aromatic, aliphatic and chlorinated hydrocarbons are usually separated from air by adsorption on activated carbon. The adsorbed solvent is regenerated with steam, after which the obtained steam-solvent mixture is condensed and water is separated from it.

However, in the case of fully or partially water-soluble solvents, the obtained condensate or condensates (aqueous organic solvent and aqueous phase containing organic solvent) must be further purified by distillation. This imposes significant additional costs on the recovery of water-soluble solvents and makes their recovery economically unprofitable.

In addition, when granular activated carbon is used without purification, the adsorption time on the carbon is limited. Therefore, the recovery of solvents at low concentrations (less than 1000 ppm) requires very large activated carbon equipment, which is technically practically impossible to implement.

2,8200 The process of the present invention uses a fibrous activated carbon fabric, such as the product marketed under the trade name "Charcoal Cloth". The use of a fibrous acti droplet in the recovery of solvents is already known from FI patent application Θ42732. Said method uses a nonwoven mat, the adsorption capacity of which is improved by applying a direct voltage to it. The activated carbon bed is supported on the sides by a separate support network. To regenerate water-soluble solvents, the fibrous activated carbon bed is heated by means of an alternating current, after which the solvent is stripped by passing a hot inert gas, preferably flue gas, from which the oxygen has been removed by combustion.

According to said application, the adsorption bed is further formed as a cylinder, whereby the flow of gas is made uniform over the entire surface area of the activated carbon layer.

The disadvantage of this method is that there is always a risk of explosion when using air containing flammable solvents, even if all possible risk factors are taken into account in the installations. Electricity from electricity is still remarkably expensive, which increases recovery costs. The network supporting the activated carbon bed increases the air resistance of the bed, which means that larger fans are needed to overcome the pressure drop.

The object of the present invention is to obviate the drawbacks associated with the prior art and to provide a completely new type of method and apparatus for recovering impurities in gases.

The invention uses a heat transfer member having a substantially cylindrical curtain surface to heat the activated carbon bed. Such solutions are already known from the patent literature. Thus, in the method disclosed in GB 218,974, the gas is introduced into a granular activated carbon-containing material which is cooled by a coolant which flows into a layer of material arranged in a piping. The activated carbon layer 3 82200 is regenerated by passing a heat transfer medium through a pipeline while blowing hot steam into the carbon layer. According to GB 227,405, the pipes used in the previous apparatus are replaced by a pipe coil through which hot steam can be passed to heat the carbon layer and water or other liquid to cool the layer.

The invention is based on the use of fibrous activated carbon in the form of a mat or fabric, which is formed into a cylindrical bed supported by a heat transfer member, in particular a heating / cooling coil.

More specifically, the method according to the invention is mainly characterized by what is set forth in the characterizing part of claim 1.

The device according to the invention is characterized in that the activated carbon layer consists of fibrous activated carbon in the form of a mat or fabric, which is formed into a cylindrical bed supported by a heat transfer member.

In the process according to the invention, the adsorption capacity of the activated carbon bed can, if desired, be improved by cooling the bed by means of a heat transfer element, whereby the recovery of even small concentrations becomes possible. The equipment can then also be used to recover solvents with a low boiling point (below room temperature), such as ethyl chloride.

Within the scope of this invention, the curtain surface of the heat transfer and support member may partially, but only partially, coincide with the actual surface of the member. The heat transfer member must be open laterally in order to allow the gas to flow substantially unimpeded through the fibrous activated carbon layer, either from outside the member into it or vice versa. The heat transfer member is then substantially open at at least one end so that air can pass through the member.

4 82200

Preferably, the heat transfer and support member according to the invention comprises a metal coil through which a heat transfer or receiving medium is passed. When a coil is used to heat the bed, the medium is preferably steam (e.g. 80-150 ° C water vapor) or a hot liquid such as water or oil. Among the oils used are paraffinic, silicone and di-n-butyl phthalate oils. If it is desired to cool the bed, water, cooling mixtures such as ice and water or mixtures of alkali metal or alkaline earth metal chlorides and water or ice, or cooling agents such as ammonia or chlorofluorocarbons are used as the medium.

The medium to be passed through the coil is usually under pressure, which is why the coil is preferably made of a metal tube with a circular cross-section. However, the pipes can be, for example, oval or rectangular in cross-section, provided that they still meet the strength requirements set by the medium used and the application.

In addition to the coil, another heat transfer element with a cylindrical curtain surface can also be used. Thus, for example, a cylindrical tubular curve with tubular loops running vertically from top to bottom and from bottom to top, respectively, is suitable for use.

It is further conceivable to use a cylinder which is open at at least one end and in the casing of which a plurality of holes extending through the wall are formed.

According to the invention, the adsorption bed is thus regenerated by heating it by means of a member having a cylindrical curtain surface, whereby the vapor pressure of the adsorbed solvents increases and the solvents are desorbed. The solvent vapors can then be flushed out of the adsorption device with compressed gas. If the adsorbed solvent is not water-soluble, e.g. an aromatic hydrocarbon, water vapor, otherwise air or an inert gas, can be used as the purge gas.

The use of large amounts of regeneration gas has been found to be futile. Due to the liquefaction of the solvent in the condenser, a pressure difference is formed between the adsorption vessel and the condenser, as a result of which the desorbed solvent vapors are subjected to the flow towards the condenser. However, a small flow of regeneration gas can ensure that not too much solvent remains in the gas space of the vessel at the end of regeneration.

The adsorption bed structure presented above is both sturdy and very air permeable (low flow resistance). According to a particularly preferred embodiment of the invention, the adsorbed solvents can be regenerated by means of a vacuum, i.e. more precisely under reduced pressure. It is then preferably operated at an absolute pressure of 1 to 90 kPa, preferably about 10 to 40 kPa. The vacuum is provided by a vacuum pump known per se, such as an oil pump or a steam ejector or a diffusion pump. The desorbed solvent vapors are passed through a vacuum pump to a condenser operating at normal pressure, where they are liquefied by means of a cooling medium, usually water. If necessary, a water separation element, such as a nitrogen trap, can be connected to the equipment.

Vacuum regeneration is particularly well suited to e.g. for the recovery of substances with a low flash point and / or substances which boil at high pressure (typically above 100 ° C) and / or substances which polymerise, decompose, etc. under the influence of heat. Substances which are completely or partially soluble in water are recovered from the condenser as immiscible in water, since no direct steam is used in the desorption.

In connection with vacuum regeneration, eg hot water or low-pressure steam is used to heat the bed.

A further advantageous feature of vacuum regeneration is: By interrupting the heating of the bed at a suitable - experimentally determinable - stage, the bed is allowed to cool adiabatically while the adsorbed substance still in the bed takes its desorption heat from the heat content of the bed. The bed is cold after regeneration and can be used immediately and efficiently for adsorption without a separate cooling step. The desorption cycle time is then significantly shortened.

The following is a summary of the main solvent groups for the recovery of which the activated carbon adsorption device having the structure according to the invention, in particular in combination with vacuum regeneration, is particularly well suited:

Chlorinated solvents which, when thermally decomposed, form e.g. corrosive HCl: 1,1,1-trichloroethane (= methyl chloroform), methylene chloride and ethyl chloride.

Easily decomposable solvents: Esters such as ethyl acetate, butyl acetate, methyl formate and ethyl formate; Ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone; Aldehydes such as formaldehyde (= formalin) and benzaldehyde; Acetals.

Low boiling point solvents (low auto-ignition point): ethyl chloride, ethyl ether, ethylene oxide and carbon disulphide.

Polymerizable solvents: styrene monomer, methyl methacrylate, ethylene oxide and propylene oxide.

High boiling point solvents (Boiling point above 180 ° C): aliphatic hydrocarbons, aromatic hydrocarbons such as toluene and pyridine and its derivatives.

The solution according to the invention achieves considerable advantages. Thus, the risk of explosion caused by electrical equipment can now be avoided. Also, since the bed is not heated directly by the water vapor blown into it, drying the bed at the end of the de-sorption step is unnecessary. The cycle time becomes shorter than in desorption with water vapor. Energy costs are reduced compared to electric heating: The cost of the medium passed through the coil is significantly lower than electrical energy. Thus, for example, the costs of using steam 7 82200 are only about 25% of the costs of electricity.

Because the heat transfer coil supports the activated carbon bed, a separate support structure is not required, and the flow of air through the ad sorption bed is facilitated.

The vacuum sorption carried out according to the invention has several additional advantages:

If the vacuum is high enough, there is not enough oxygen in the desorption stage that the solvent vapors could ignite. This increases safety, especially in the recovery of substances with a low flash point. By lowering the operating pressure, the desorption temperature is lowered, whereby the amount of desorption energy required is reduced. Avoid the use of expensive hot oil systems. Compared to other regeneration methods, the lower desorption temperature reduces the thermal decomposition or polymerization of the adsorbed material or other similar undesired reactions. This avoids the investment and operating costs involved in separating the reaction products from the recovered solvent or eliminating the disadvantages caused by the reaction products, e.g. by adding a neutralizing or stabilizing agent to the recovered solvent.

Substances which are completely or partially soluble in water are recovered from the condenser without mixing with water, since direct steam does not have to be used in the desorption. Most of the de-sorbed material leaves the system as a fairly clean and concentrated gas stream sucked in by a vacuum pump. Compared to inert gas sorption, the absence of non-condensable gases is determined from the condensation with a lower heat transfer element and operating costs. This contributes to compensating for the investment costs of the vacuum pump.

In the following example, the invention will be examined in more detail with the aid of the accompanying drawing, which shows in principle a cross-section of a device according to the invention.

8 82200

The device according to the invention comprises a heat transfer coil 1, as well as a layer of fibrous activated carbon 2 wrapped around the coil. The coil can be made of any suitable metal which conducts heat well and is not susceptible to corrosion by impurities in the air to be cleaned. Suitable for use are e.g. various stainless and acid-resistant steel tubes and copper and copper alloy tubes.

To provide a large heat transfer surface (and a good support surface), the helix must have a sufficient number of threads. The number of threads in the height direction of the helix is preferably 5 to 30, preferably 10 to 15 per meter.

The other end of the coil 1 can be connected to a source of heat transfer medium on the one hand and to a source of heat receiving medium on the other hand, which are not shown in the figure. Steam (pressurized water vapor) is usually used as the heat transfer medium, and water or a mixture of salt and water and / or ice is used as the heat transfer medium. The medium is passed either once or, preferably, recirculated several times through the coil.

The fibrous activated carbon 2 is preferably in the form of a fabric or mat, making it easy to handle. One preferred product is the elastic nonwoven fabric "Charcoal Cloth" supplied by Charcoal Cloth Limited, which has fibers with good adsorption capacity, consisting almost 100% of activated carbon. At least 1, preferably 5 to 15, layers of activated carbon are arranged around the helix 1. The fibrous activated carbon layer can, of course, be reinforced, if desired, with e.g. porous natural fibers or synthetic fabrics. This measure is often not necessary in the solution according to the invention, and by impeding the gas flow, it may even be detrimental to the operation of the gas purifier.

The fibrous activated carbon bed 2 is installed in a tank 3 having a solvent-containing air inlet pipe 4, 9 82200 compressed air outlet pipe 5 and regeneration gas inlets outlet pipes 6 and 7. The second end of the cylindrical fibrous activated carbon 2, i.e. its upper end, is closed in the figure. 8 while the other end, i.e. the lower end, is open and surrounds the mouth of the purified air outlet pipe 5. The length of the activated carbon cylinder 2 is at least somewhat smaller than the inner height of the tank 3, whereby the air supplied from the inlet pipe 4 can pass the airtight end of the cylinder. The air to be treated is thus led from the inlet pipe 4 to the tank 3 and further through the bed so that the flow takes place through a tube between the outer surface of the fibrous activated carbon cylinder 2 and the inner surface of the tank 3 into the cylinder and from here to the outlet duct 5. In the invention 2, through the activated carbon wall and removed from the tank via pipe 4. The choice of this flow direction is particularly important when cooling the bed 2 by means of the screw 1 in order to improve its adsorption capacity. The coil l then cools the air at the same time before it reaches the activated carbon, which reduces the total need for cooling power.

The air is preferably passed only once, but if necessary, it can of course also be recirculated to increase the efficiency of the adsorption process.

To regenerate the activated carbon bed, the bed is heated by means of the coil 1 while air, inert gas (nitrogen, argon, deoxygenated flue gas) or steam is introduced into the tank via the inlet pipe 6. In the solution according to the invention, it is not necessary to use warm regeneration gas, but the medium to be passed through the heat transfer coil heats not only the bed 2 but also the gas flowing from the inside to the outside. The solvent-containing regeneration gas is passed to a condenser where the solvent and any other impurities are separated.

In an alternative embodiment, the supply pipes 4, 5 of the air to be cleaned are closed by means of valves 10, not shown, and a vacuum is sucked into the tank by means of a vacuum pump which can be connected to the pipe 7. The regeneration gas supply pipe 6 is not actually required in this embodiment. However, if such a pipe is installed in the tank to allow alternative regeneration methods, it is also closed by a valve during vacuum regeneration.

There are preferably at least two adsorption units having a structure according to the invention in the gas recovery plant, one of which is regenerated while the other is in the adsorption shift.

In the exemplary case, the heat transfer coil 1 was made of acid-resistant steel with a diameter of 40 cm, a height of 80 cm and a pitch of 12 cm. Wrapped around the coil 1, 10 layers of fibrous activated carbon mat ("Charcoal Cloth") totaling 1.1 kg.

Example 1:

The above device recovered isopropanol (IPA) from the industrial exhaust air. Adsorption was performed both by cooling the adsorption bed and without cooling. The following are the experimental results of the air purification: a) Adsorption step: 1. The inlet gas flow was 480 m1 2 / h, the flow through the bed perpendicular was 0.30 m / s. The gas contained 450 mg / m2 of isopropanol. The fibrous activated carbon bed was not cooled. The isopropanol content of the exhaust gas was 360 mg / m2, purification yield = 20%. The dynamic capacity of fibrous activated carbon is 4.3%.

Il

Supply air flow and composition the same as above. A calcium chloride sol with a temperature of + 5eC was passed through the coil. The solvent content of the exhaust air was 254 mg / m2, the air purification yield was 88%. Dynamic capacity of fibrous activated carbon 19%.

11 82200 (b) Regeneration phase:

The isopropanol-containing activated carbon bed according to item 2 was regenerated as follows: Steam was introduced into the coil from about 130 ° C, which heated the fibrous activated carbon bed so that the Boiling temperature of IPA was reached. To flush the isopropanol from the tank, warm air was introduced into it, whereupon it was distilled off, and the resulting air-IPA mixture could be concentrated in a condenser for reuse. 178 g of isopropanol were recovered.

Example 2:

1,1,1-Trichloroethane was recovered with the device according to the invention, the experimental results being as follows: a) Adsorption step:

The inlet gas flow was 600 m3 / h, the flow through the bed perpendicular to 0.20 m / s. The amount of 1,1,1-trichloroethane was 20 g / m3. The fibrous activated carbon bed was not cooled. The trichloroethane content of the exhaust gas was 0.5 g / m3. The dynamic capacity of the fibrous activity charcoal was 20%.

b) Regeneration phase:

The adsorption bed was regenerated by suction under reduced pressure. The pressure level was 30 kPa absolute. The bed was heated by passing water from 90 ° C through the coil.

1,1,1-Trichloroethane was removed from the system via a vacuum pump to a condenser operating at normal pressure, where it was liquefied by the indirect action of cooling water.

No increase in acidity or other indications of thermal decomposition of the recovered 1,1,1-trichloroethane could be observed.

The adsorption-desorption cycle was repeated several times with the same batch of solvent, but even with repeated use, no signs of decomposition could be detected, unlike e.g. water vapor recovery based on regeneration.

Claims (5)

12 82200 A method for recovering impurities in gases, in particular solvents, which comprises - separating the compounds by adsorbing them on activated carbon (2), - desorbing the compounds by raising the temperature of the activated carbon layer (2), and - heating or cooling the activated carbon layer (2) substantially by means of a heat transfer member (1) having a curtain surface, through which a heat transferring or receiving medium is passed, characterized in that - activated carbon is a fibrous activated carbon in the form of a carpet or fabric formed by a cylindrical bed (2) supported by the heat transfer member (1) . Method according to Claim 1, characterized in that a vacuum is applied to the bed (2) during heating, preferably an absolute pressure of 1 to 90 kPa, particularly preferably 10 to 30 kPa, for desorbing the adsorbed compounds. A method according to claim 2, characterized in that the heating of the bed (2) is stopped at least somewhat before all the adsorbed substance has been desorbed to cool the bed at the end of the desorption step. An apparatus for recovering impurities in gases, in particular solvents, comprising: - a tank (3), - a layer of activated carbon (2) arranged inside the tank (3), - a purge air inlet pipe (4) and a purified air outlet pipe (5), - a tank ( 3) combined means (6, 7) for regenerating the activated carbon layer (2) and a substantially helical heat transfer member (1) arranged in connection with the 13 82200 layer (2) for heating or, if necessary, cooling the activated carbon layer, characterized in that - the activated carbon layer (2) ) consists of fibrous activated carbon in the form of a mat or fabric formed as a cylindrical bed supported by a heat transfer member (1). Device according to Claim 4, characterized in that the activated carbon in the form of a fibrous carbon cloth or mat (2) is wrapped around the heating element. 14 82200